The present invention relates generally to a hydraulic chain tensioner having a lower piston and an upper piston, both longitudinally movable in a housing. The present invention has particular application to a cushioned stop mechanism for such a tensioner.
Tensioning devices, such as hydraulic tensioners, are used as control devices for power transmission chains as a chain travels between a plurality of sprockets. In an automotive application, the tension of the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. As a result, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or unmeshing of the chain with the sprocket teeth. It is especially important in the case of a chain-driven camshaft in an internal combustion engine to prevent the chain from slipping because the camshaft timing can be misaligned by several degrees, possibly rendering the engine inoperative or causing damage.
In a typical hydraulic tensioning device with a ball-type check valve, fluid flows from a pressurized fluid source through a clearance formed between the ball and the seat of a check valve. The hydraulic pressure from an external source, such as an oil pump or the like, flows into a chamber through passages formed in the housing, easily moving the piston outward by the combined efforts of the hydraulic pressure and the spring force.
On the other hand, when the piston tends to move in a reverse direction, the ball of the check valve is tightly contacted with the ball seat to restrict the flow of fluid from the chamber, thereby preventing retraction of the piston. In this manner, the tensioner achieves a so-called no-return function, i.e., are easy in one direction (outward) but difficult in the reverse direction (inward).
A potential problem with hydraulic tensioners of this construction, however, is that they may not always maintain a predetermined tension, especially when an engine is being started or idling at rest with little or no oil pressure. Unless appropriate oil pressure is applied to the chamber, or the chamber is filled with a sufficient amount of oil, the piston moves easily in both directions and loses the no-return function, resulting in noises and vibrations in the chain and associated mechanisms during start-up conditions.
A solution to this potential problem is to provide the tensioner with a rack and ratchet assembly to act as a mechanical no-return device. U.S. Pat. No. 5,346,436 to Hunter et al., which is owned by the assignee of the present application and which is incorporated herein by reference, discloses a rack and ratchet assembly that provides a mechanical no-return function. A drawback of such rack and ratchet assemblies is the device usually has a certain amount of backlash, or backward movement, causing the tensioner piston to impact it during normal operation. This can cause excessive wear on the rack assembly. In addition, as engine speed and oil pressure rise, the tensioner can advance too far, over tensioning the chain and causing excessive noise or premature chain failure.
Another example of a tensioner with a ratchet mechanism is shown in Shimaya, U.S. Pat. No. 5,073,150. In Shimaya, a hydraulic tensioner is described which has uses a ratchet with a plunger to prevent excessive slackening of a chain following sudden increases in chain tension. A chamber is formed between the top of the plunger and the tensioner fixed housing. Increasing fluid pressure in the chamber decreases the force applied by the tensioner plunger to the chain. In other words, fluid pressure in the chamber is used to apply a force to the tensioner which opposes the force tending to apply tension to the chain. In all embodiments, the ratchet acts on the plunger thereby preventing retraction of the plunger into the bore of the housing.
The present invention uses engine oil pressure in a hydraulic rack tensioner to reduce impact loading and associated wear. In particular, the tensioner of the present invention includes an lower piston and an upper piston. A pawl or ratchet engages a rack formed on the lower piston when chain tension causes the upper piston to be forced downward toward the lower piston. A fluid chamber formed between the upper and lower pistons provides a fluid cushion to reduce the impact of the pawl and the rack. When the engine is producing little or no pressurized fluid, such as when the engine is idling or turned off, the piston spring acts on the lower piston and causes the rack to be advanced to the next extended setting.
In accordance with one embodiment of this invention, a hydraulic rack-style tensioner includes two concentric pistons in the bore of a tensioner housing. A first piston which occupies a generally lower position in the housing (and may also be referred to as the first, outer or lower piston) has conventional ratchet teeth on an outside surface forming a rack. The ratchet teeth formed around the circumference of the lower portion of the outside wall of the lower piston, but may be formed on one side of the piston. In this case, the tensioner and/or piston must be designed to prevent rotation in the bore.
The rack teeth are engaged by a pawl. The pawl is held against the first piston by a spring, so that as the lower piston extends, the pawl will prevent it from moving back. The lower piston is spring loaded so that the piston naturally tends to advance to the next ratchet setting.
A lower chamber is formed between the hollow lower portion of the lower piston and the bore of the housing. In this embodiment, the lower chamber is not supplied with pressurized fluid. A vent is provided in the housing opening into the lower chamber which allows air and oil to enter and exit the lower chamber unimpeded.
The lower piston has a section of reduced diameter, or a narrow waist section along the midsection of the piston, and pressurized oil is fed into the cavity between the outside of the piston and the bore of the housing. The cavity is formed on the lower piston on a side generally opposite the rack. In an alternate embodiment, the cavity may extend around the circumference of the lower piston generally above the rack portion of the piston. Oil is then fed from the cavity into a second or fluid chamber formed between the lower and an upper or second piston, the upper piston being located in a piston bore formed in an upper section of the lower piston. The oil pressure in the fluid chamber causes the two pistons to be forced apart.
The pressurized fluid forces the upper piston against the tensioner arm which is in turn contacts an associated chain. While the lower piston is biased in an outward direction by a piston spring in the lower chamber, the upper piston in this embodiment does not have a piston spring associated therewith. Movement of the upper piston in the outward direction is caused by the lower piston contacting the upper piston directly and/or pressurized fluid in the fluid chamber. The force of the chain tension on the tensioner arm, and thus on the upper and lower piston, forces the pistons back toward the tensioner when the chain force exceeds the force of the piston spring and the fluid pressure on the upper and lower pistons and causes engagement of the rack on the lower piston against the pawl. The oil pressure additionally provides a cushioning and damping effect, reducing the loads on the pawl.
A second embodiment is similar to the first embodiment with the main difference being a rack portion of the lower piston being formed about the entire circumference of the lower portion of the lower piston. Above the lower portion of the lower piston a narrow cross section of the lower piston forms a cavity with the bore of the housing which extends about the entire circumference of the lower piston.
A third embodiment uses two separate pistons stacked axially in the same bore. Functionally, this is similar to the concentric piston design. In this embodiment, a hollow lower piston is slidably received in a bore formed in the tensioner housing. The hollow portion of the lower piston forms a lower chamber with the tensioner housing. The lower chamber is vented to atmosphere. A hollow upper piston is slidably received in the same bore of the housing directly above the lower piston. In contrast to the first embodiment, both the lower and upper pistons have a piston spring which act to bias each respective piston in a protruding direction from the bore of the housing.
The upper piston in the stacked piston embodiment has a narrow waist or midsection which forms a cavity or passage between the bore of the housing and the outside of the upper piston. A opening in the midsection of the upper piston adjacent the cavity permits pressurized oil to enter a fluid chamber formed by the hollow upper piston and the top of the lower piston. The pressurized oil in the fluid chamber acts to push the upper piston in a protruding direction and the rack of the lower piston downwardly against the pawl and provides a fluid cushion between the upper and lower piston.